I don’t believe Scripture ever takes that approach, frankly. Paul taught that where "sin abounded, grace abounded all the more". Scripture doesn’t teach "Sin, so grace may abound!" but it DOES teach that grace abounds!
You have to confess and forsake sin in order to have mercy (See Proverbs 28:13, 1 John 1:9, 1 John 1:7). One unconfessed grievous sin (like lying, lusting, hating, etc.) can condemn a believer. Ananias and Sapphira is one example (Acts of the Apostles 5:1-11). For a great fear of God came upon the church when they died for having lied to the Holy Ghost. The emotion of fear (by believers) makes no sense if they were never saved and neither would such an emotion make sense if they were secure in their salvation. For if you believe future sin is forgiven you, then why work out your salvation with fear and trembling as per Philippians 2:12? I mean, why all the trembling if Philippians 2:12 is not talking about fear?
As for Romans 5:20:
Well, while Romans 5:20 says grace abounds, you also have to read Romans 5:21 that says grace reigns (rules) through righteousness (righteous living).
The grace of God teaches us to deny ungodliness, and that we should live righteously and godly in this present world (Titus 2:11-12).
Ephesians 5:25-27 says that the reason or purpose Christ died for us was so to sanctify and cleanse us with the washing of the water of the Word (Scripture) so that He may present to Himself a church that is holy and without blemish.
You said:
Paul also has a terse reply to those who slander believers in the gospel of grace:
"And why not do evil that good may come?"—as some people slanderously charge us with saying. Their condemnation is just."—Romans 3:8
Sorry, things are not as you say. The condemnation of a person who believes salvation is: "
God's Grace Through Faith + Works of Faith" in the fact that they are following Jesus (or His commands as a part of eternal life) is NOT the context.
The context is to the Jew and or trying to be justified by the whole of the 613 laws of the Law of Moses (without God's grace through faith) because Paul was fighting against "Circumcision Salvationism." How so?
Romans 3:1 says,
"What advantage then hath the Jew? or what profit
is there of circumcision?"
"Circumcision Salvationism" (i.e. Law Alone Salvationism without God's grace) was a heretical belief at the time that basically said that a man of God had to first be circumcised in order to be initially and ultimately saved. It was a false belief that was pushed by a certain sect of Jews who were trying to deceive some Christians into being justified by the Old Law (without God's grace through faith being the entrance gate and ultimate way we are saved). This was a heresy that was clearly addressed at the Jerusalem council (See Acts of the Apostles 15:1, Acts of the Apostles 15:5, Acts of the Apostles 15:24).
Paul also addressed this problem; Paul said to the Galatians that if you seek to be circumcised, Christ will profit you nothing (Galatians 5:2), and then Paul mentions how if you seek to be justified by the Law, you have fallen from grace (Galatians 5:4). This "law" is the Torah because circumcision is not a part of the commands given to us by Jesus and His followers. In fact, even under the Old Covenant (before the cross), the requirement of circumcision did not even work in this way. Romans 4:9-12 says that Abraham was justified by faith (belief) in God before he was circumcised. So these Jews were trying to deceive Christians into being circumcised got the order all wrong. They were trying to make salvation all about the Law Alone without God's grace as the entrance gate and foundation of one's faith.
So the context of Romans 3:8 that says, "
And why not do evil that good may come?"—as some people slanderously charge us with saying. Their condemnation is just" are not Christians like me who believe we must believe the words of Jesus in Luke 10:25-28 (whereby Jesus agrees with the lawyer on the truth that we must love GOD and love our neighbor as a part of inheriting eternal life), but the context is "Circumcision Salvationism" (Which is alluded to in Romans 3:1).
The problem with saying future sin is forgiven a person is that:
#1. The Bible never teaches clearly that future sin is forgiven a person.
#2. Various verses teach that believers still have sin that needs to be forgiven (Like 1 John 1:9, etc.). For if future sin is forgiven you, then how can one confess of sin in order to be forgiven of sin? It makes no sense.
#3. It turns God's grace into a license for immorality (Jude 1:4). For how do you even explain Jude 1:4? Do you just like not think about Jude 1:4? What about 2 Peter 2:1, and 2 Peter 2:14? It says that there were false prophets who could not cease from sin. The Eternal Security Proponent or the Sin and Still Be Saved Type Believer tells me that they can never overcome grievous sin (like lying, lusting, hating, etc.) in this life and that they will always occasionally stumble into sin. Yet, how do they explain 2 Peter 2:1, and 2 Peter 2:14? Do they just ignore such verses? Do they re-interpret them because they do not like what they say? If their belief is to be true, they must do so. They must alter or ignore Scripture verses like these in order to make their belief that says that they can sin and still be saved work.
For if you believe that a saint can die in unconfessed grievous sin (like lying, lusting, hating, etc.) and still be saved, and you also believe that you will always fall into some kind of sin (Which is the OSAS song of many) my claim towards the OSAS Proponent belief is not slanderous in any way but it is merely the truth. For they are saying they can sin on some small level again and still be saved. I have heard it time and time again.
Now, if you believe (like one of my close friends) who thinks that in order for OSAS to be true, one cannot justify even one grievous sin, but they must confess and forsake it, then that would be different. But even this kind of belief is wrong because it doubts the promise of GOD (that you are truly forgiven when you come to the Lord), and it doubts the warnings that a servant of the Lord can fall away.
Side Note:
Please take note that I am aware that there are those who do not believe in OSAS (Once Saved Always Saved) and yet they also believe they can sin and still be saved on some level. Free Will Baptists would be one example of this. Some believe you can only sin a little bit on occasion as long as one strives to live a holy life (Whatever that means) and others believe you can sin as much as you like and still be saved. I have talked with believers who thought they could mow down a crowd with sub machine gun and they would still be saved while doing so all because they have a belief on Jesus. George Sodini was a huge proponent of Eternal Security or OSAS and he wrote in his suicide letter that he would be saved despite his mass murder and the taking of his own life. That's the problem I have with this kind of belief. It turns God's grace into a license for immorality as per Jude 1:4. There are different levels of justifying sin and evil. To say to a person their future sin is forgiven them and then tell them nothing about how they must live holy can create yet another potential George Sodini.
To read up on George, check out this article here:
George Sodini.
